The research is focused on defining the cellular and molecular biology of the hepatic and pancreatic stem cell compartments in normal and neoplastic organs with a particular emphasis on the role of growth factors and cytokines. Leukemia inhibitory factor (LIF) is a polyfunctional cytokine that is known to induce acute phase proteins in the hepatocytes. No comprehensive study has yet been performed on the physiological role of this cytokine during liver regeneration. Thus, we studied the level of expression and cellular distribution of transcripts for LIF, its receptor (LIFR), and signal transducing subunit gpl3O during rat liver regeneration after both simple partial hepatectomy (PH) and the oval cell activation induced by the combination of 2-acetylaminofluorene and PH. In addition, the expression of an acute phase protein alpha1-acidglycoprotein was examined. The level of transcripts for LIF and its receptor subunits increased and remained elevated during oval cell expansion. In contrast, after PH, the transcripts were induced only transiently, showing a peak 24 hours after the operation. LIF and receptor subunits were expressed in both parenchymal and nonparenchymal fractions in the 2-acetylaminofluorene/PH model, but the level of expression was most pronounced in the nonparenchymal fraction. In situ hybridization clearly revealed a strong expression of LIF, LIFR, and gpl3O in the oval cells and demonstrated only a weak expression in the parenchyma. Interestingly, transcripts of alphal-acidglycoprotein were exclusively detected in the parenchyma. These results suggest a phenotypic difference between oval cells and hepatocytes in their signaling through gpl30. We hypothesize that the LIF/LIFR gpl30 system may be involved in the expansion and differentiation of the hepatic stem cells.